E:\work\qt\build-myFirstOpenCVProject-Desktop_Qt_5_7_1_MinGW_32bit-Release\releaseE:\work\qt\build-myFirstOpenCVProject-Desktop_Qt_5_7_1_MinGW_32bit-Release\release\myFirstOpenCVProject.exe does not seem to be a Qt executable

@Vics You are using the wrong version of Qt dlls. This happens when the dll that is loaded is built by a different compiler. I.e. you have a Qt5Core.dll on your system in your system path that is built by MSVC and then you built (say using Qt Creator) a Qt5Core.dll that was built with mingw. So your app uses the mingw one but when you run it, it picks up the MSVC version.

Since this scenario is not compatible you get that can not find entry point error.

The windeployqt thing can fix it as it will put the correct dlls in the exe path which will be used above the system ones. Just wanted to explain in depth why this was happening so you could fix it manually (without windeploy) if you so chose.

I checked my comp for Qt5Core.dll.
The results were very intresting becouse Qt5Core.dll discovered in CMake folder. This folder was set in Path system variable.
During installing CMake this folder was added to Path variable.

I removed this folder from Path. Now my program working without QtCreator.